Seamless steel tube for use as a steel catenary riser in the touch down zone

ABSTRACT

The present invention describes an upset SCR of novel low carbon chemical composition and microstructure as well as method of manufacturing the same, which achieves higher improvement in the fatigue life as it is integral with the riser pipe section at the Touch Down Zone. The low carbon upset SCR achieves its desired properties by the thermal treatment which it is subjected to. The novel low carbon chemical composition and microstructure comprises in weight per cent, carbon 0.04-0.10, manganese 0.40-0.70, silicon 0.15-0.35, chromium 0.40-0.70, molybdenum 0.40-0.70, nickel 0.10-0.40, nitrogen 0.008 max, aluminum 0.010-0.045, sulfur 0.005 max, phosphorus 0.020 max, titanium 0.003-0.020, niobium 0.020-0.035, vanadium no more than 0.10, copper 0.20 max, tin 0.020 max, and carbon equivalent 0.43 max and PCM no more than 0.23 and having a yield strength of at least of 65000 psi, the ultimate tensile strength of at least 77000 psi and YS/UTS ratio below 0.89 in material representing the pipe body, the transition zone and the upset end.

FIELD OF THE INVENTION

This invention relates to seamless steel tubes for use as steel catenaryriser.

BACKGROUND OF THE INVENTION

In recent years, the interest in exploiting deeper water off-shoreoilfields has increased sensibly. As a consequence, various solutions ofmarine production systems have been developed. The currently-availablesolutions are directed generally to semi-floating and floatingproduction systems which are subjected to various movements with respectto seabed, mainly due to marine waves, currents and tides phenomena. Theaforementioned systems are complemented by compliant riser systemscompatible with mobile surface stations.

Steel Catenary Risers (SCR) represents one of the most outstanding risersystems to be adopted in these challenging situations. Such component isnormally subjected to complex spectra of fatigue loading related to boththe mobility of the floating platform and to the very large free span ofunconstrained line from the seabed to surface. As a consequence, a bigconcern in the design of a SCR is related to the fatigue resistance.Since the cyclic loading is predominately in the axial direction, itdirectly stresses the welded joints between abutting pipes. Such jointsgenerally represent the weakest point with respect to fatigueresistance, and the design life of the whole riser is determined by thecapability of this component to resist the fatigue loading.

Steel Catenary Riser is a proven and economic riser system solution, asa tie-back production riser and as an export riser from FloatingProduction Systems (FPS) in the development of oil and gas fields indeepwater and ultra-deepwater. The application of SCRs is challenged by,in some cases, the high fatigue damage at the Touch Down Zone (TDZ) froma combination of field specific parameters such as riser size, fluidcharacteristics, vessel motions, metocean parameters, soil conditions,and water depth.

The most severe design requirement for SCRs is the fatigue life of thegirth welds at the Touch-Down Zone (TDZ) region where the riser touchesthe seafloor and it connects with the rest of the pipeline, as isillustrated in FIG. 1. In this zone, the riser experiences the highestlevel of cumulative fatigue damage. This is due to the fact that in saidzone the highest bending of the catenary line is experienced, contraryto the total absence of bending of the portion of the line lying on theseabed. Due to the various movements of FPS (waves, tides, currents,etc.), the line segment in the TDZ experiences cycles of bending betweenmaximum riser bending and zero bending (straight). The severity offatigue loading in the TDZ is further complicated by the presence ofcontinuous impacts of the portion of the line when entering in contactwith the ground. Moreover, it has to be considered that the same impactof the line could dig a hole just in correspondence of the TDZ,amplifying the amplitude of bending cycle.

In other words, constant motion by the topside floating vessel resultsin cyclic pounding for the riser against the sea-floor that, if notproperly designed, can result in fatigue failure. In addition to theriser motion, other factors that can increase the severity of the TDZfatigue include large pipe diameter, deep water depth, high currents,and sour service (corrosion degradation).

Various possible solutions for the improvement of fatigue life at SCRTDZ have been devised and studied. SCRs are utilized as riser systems inongoing semi-submersible projects.

Alternative options to obtain an increase in fatigue life at the TDZ inthe deepwater field developments have been devised to enable the use ofSCR. These solutions include: ID machining for better fit-up and the useof improved welding techniques; the use of thick forged ends weldedonshore to ensure better fit-up and to reduce the Stress ConcentrationFactor (SCF); the periodic movement of the floating vessel to distributefatigue damage over longer length at the TDZ; and the use of clad steel.

The upsetting process is commonly used in the industry for casing andriser joints with threaded ends. Steel grades with higher carbon contentare normally used for these applications. The upsetting process has notbeen used so far for weldable pipe of SCR quality. In most of threadedcases, though, the increase in fatigue life has been limited to a factorbetween 2 to 3. In the case of clad steel applications, a higherincrease in fatigue life can be achieved. In addition, alternativecatenary riser design has been developed by changing the riser pipematerial (composite, titanium) or by hybrid designs (titanium andsteel), or by changing the shape near seabed through provision ofsignificant buoyancy (WO97/06341).

The alternative designs have focused on the improvement of the catenaryriser strength near and above the seabed, thus enabling their use inharsher environment and more challenging applications.

Thus, there is a need to enhance conventional Steel Catenary Risers forTouch Down Zone (SCR TDZ) design for achieving significant increase infatigue life, particularly, increasing fatigue life in SCR TDZ above 3under sour and non-sour service environments with a pipe consisting ofthree regions: pipe body, transition zone and upset end, as isillustrated in FIG. 2.

To accomplish this need, upset pipes to be used in welded joints havebeen developed. The simple concept for the improved fatigue performanceconsists, in this case, in locally decreasing the stress experienced bythe welding with respect to the stress range generally experienced bythe pipe body and, hence, the section of the riser in the TDZ. An upsetSCR of novel low carbon chemical composition and microstructure was thusdevised to achieve higher improvement in the fatigue life as it iscomprised with the riser pipe section.

The feasibility of manufacturing thick upset end Riser for the TouchDown Zone with improved fatigue resistance varies, nevertheless, withthe grade of steel that can be welded for offshore applications. Thefeasibility to manufacture a thick end Riser for the Touch Down Zonewith improved fatigue resistance is the key to ensure that the upset SCRhas practical value in application at the TDZ.

Several Steel Catenary Riser (SCR) solutions have included mild sourservice requirements. Sour Service is the performance of the Riser inH₂S environments. Metallurgical properties known to affect performancein H₂S containing environments include: chemical composition, steelcleanliness, method of manufacturing, strength, amount of cold work,heat treatment conditions and microstructure. Since the upset pipemanufacturing process involves additional steps subsequent to themanufacture of the seamless pipe, the end product has to accomplishthese requirements.

BRIEF DESCRIPTION OF THE INVENTION

The present invention describes an upset SCR of novel low carbonchemical composition and microstructure which achieves higherimprovement in the fatigue life as it is integral with the riser pipesection at the Touch Down Zone. The low carbon upset SCR achieves itsdesired properties by the thermal treatment which it is subjected to.The novel low carbon chemical composition and microstructure comprisesin weight per cent, carbon 0.04-0.10, manganese 0.40-0.70, silicon0.15-0.35, chromium 0.40-0.70, molybdenum 0.40-0.70, nickel 0.10-0.40,nitrogen 0.008 max, aluminum 0.010-0.045, sulfur 0.005 max, phosphorus0.020 max, titanium 0.003-0.020, niobium 0.020-0.035, vanadium no morethan 0.10, copper 0.20 max, tin 0.020 max, and carbon equivalent 0.43max and PCM no more than 0.23 and having a yield strength of at least of65000 psi, the ultimate tensile strength of at least 77000 psi andYS/UTS ratio below 0.89 in material representing the pipe body, thetransition zone and the upset end.

The present invention also describes a method for manufacturing aseamless steel tube for steel catenary riser with upset ends having ayield strength at least of 65000 psi both in the pipe body, transitionand the upset-zone comprising the steps of: (a) providing a steel tubecomprising in weight per cent, carbon 0.04-0.10, manganese 0.40-0.70,silicon 0.15-0.35, chromium 0.40-0.70, molybdenum 0.40-0.70, nickel0.10-0.40, nitrogen 0.008 max, aluminum 0.010-0.045, sulfur 0.005 max,phosphorus 0.020 max, titanium 0.003-0.020, niobium 0.020-0.035,vanadium no more than 0.10, copper 0.20 max, tin 0.020 max, and carbonequivalent 0.43 max and PCM no more than 0.23; (b) upsetting the tubeends in multiple steps with intermediate heating cycles in between toachieve the required thickness (c) quenching and tempering between630-710° C.; (d) machining the upset ends.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the Steel Catenary Riser Configuration of a preferredembodiment of the present invention.

FIG. 2 illustrates an embodiment of the tube with an upset end of apreferred embodiment of the present invention.

FIG. 3 shows typical macro sections of RP2Z welds for different weldingconditions of the tube of a preferred embodiment of the presentinvention.

FIGS. 4( a) and (b) show the tensile test results for the longitudinaldirection and the transverse direction of a preferred embodiment of thepresent invention.

FIG. 5 illustrates the longitudinal and transverse Y/T ratio results ofa preferred embodiment of the present invention.

FIG. 6 shows the Hardness Vickers HV10 of a preferred embodiment of thepresent invention.

FIG. 7 illustrates the Transverse Charpy V Notch Impact Test at −30° C.of a preferred embodiment of the present invention.

FIG. 8 shows the Mean curve for specimens 10¾″×0.866″×65 of a preferredembodiment of the present invention.

FIGS. 9( a) and (b) show the tensile test results for the longitudinaldirection and the transverse direction of a preferred embodiment of thepresent invention

FIG. 10 illustrates the longitudinal and transverse Y/T ratio results ofa preferred embodiment of the present invention.

FIG. 11 shows the Hardness Vickers HV10 of a preferred embodiment of thepresent invention.

FIG. 12 illustrates the Transverse Charpy V Notch Impact Test at −30° C.of a preferred embodiment of the present invention.

FIG. 13 shows the Mean curve for specimens 10¾″×1.250″×65 of a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in FIG. 1, the present invention describes an upset SCRof novel low carbon chemical composition and microstructure whichachieves higher improvement in the fatigue life as it is integral withthe riser pipe section at the Touch Down Zone. The low carbon upset SCRachieves its desired properties by the thermal treatment which it issubjected to.

The steel grade contemplated for use in the upset SCR of the presentinvention is X-65 (a yield strength of at least 65000 psi in the pipebody and in the upset ends).

The alloy design consists of a low-C (0.13 max), low-Mn (1.5 max) steelwith additions of microalloying elements such as Niobium, Titanium(Nb+Ti 0.1 max), Chromium and Molybdenum (Cr+Mo 1.2 max). The purpose ofadding these last two alloying elements is to increase hardenability andpromote a martensitic-bainitic transformation on thick upset ends andpipe body achieving high strength. The carbon equivalent (CE) isdesigned not to exceed 0.43 as requested by API 5L. More preferably, thecarbon equivalent is limited to 0.41. The most preferred embodiment ofthe present invention is not to exceed 0.39.

Pipes are hot rolled using a recrystallization controlled rolling schememanufactured from round billets obtained by continuously cast (CC)process. After hot rolling, the pipes are then inspected withnon-destructive methods such as electromagnetic inspection, wet magneticparticle inspection and ultrasonic testing with the purpose of findingany longitudinal or transverse defects on internal or external surfacesand to verify wall thickness. The pipes are then upsetted by reheatingthe pipe ends above the dissolution temperature of Nb (C, N) to provideadequate plastic flow during each upset operation whilst controllingaustenite grain size by precipitation of fine TiN particles. The optimumradius at the upset-pipe body transition is modeled thru Finite ElementAnalysis (FEA), where the Stress Concentration Factor (SCF) resulted1.135 and 1.12 for Case 1 (273.1 mm OD by 22.0 mm WT Pipe body, 28 mm WTas machined Upset Ends and 35 mm as upset ends, steel grade X65 fornon-sour service application, 10.75″×0.866″) and Case 2 (273.1 mm OD by31.8 mm WT Pipe body, 45 mm WT as machined Upset Ends and 53 mm as upsetends, steel grade X65 for sour service application, 10.75″×1.250″),respectively. After upsetting both ends of the pipes, a critical quenchand temper heat treatment is designed and used to provide the finalmechanical properties. Non-destructive testing is again carried out inthe pipe body, and the OD and ID surface of the upset ends are machinedand then inspected with wet magnetic particle inspection and manualultrasonic testing. Finally, the pipes are bevel machined for girthwelding. Welding and fatigue behavior are thoroughly characterized.

After the quench and temper heat treatment, the material is then fullycharacterized. The Yield Strength (YS), the Ultimate Tensile Strength(UTS) and the YS/UTS ratio at room temperature are evaluated using bothlongitudinal and transverse round specimens taken from the Upset End,Slope Transition and Pipe Body regions in two quadrants, 0° and 180°.

Hardness Vickers HV10 are measured on the OD (outside diameter), MW(mid-wall) and ID (internal diameter) sections in 4 quadrants are takenfrom the Upset End, Slope Transition and Pipe Body regions. The hardnessreadings are taken at 1.5 mm from OD and ID. In addition, transverseCharpy V notch impact testing is carried out at −30° C. and −40° C. forcase 1 and case 2 respectively using 10×10 mm specimens. Sour serviceresistance is assessed in both pipe body and upset ends by the HydrogenInduced Cracking (HIC) and Four Point Bend Tests (FPBT).

The present invention thus describes a seamless steel tube for a steelcatenary riser with upset ends comprising in weight per cent, carbon0.04-0.10, manganese 0.40-0.70, silicon 0.15-0.35, chromium 0.40-0.70,molybdenum 0.40-0.70, nickel 0.10-0.40, nitrogen 0.008 max, aluminum0.010-0.045, sulfur 0.005 max, phosphorus 0.020 max, titanium0.003-0.020, niobium 0.020-0.035, vanadium no more than 0.10, copper0.20 max, Tin 0.020 max, and carbon equivalent 0.43 max and PCM no morethan 0.23 and having a yield strength of at least of 65000 psi inmaterial representing the pipe body, the transition zone and the upsetend.

The novel microstructure of the upset SCR which enables the seamlesssteel tube to achieve a higher improvement in the fatigue life includesthe following mechanical properties and corrosion requirements for UpsetSCR as shown in Table 1. The minimum requirements are following the API5L, 43^(rd) edition specification.

TABLE 1 Mechanical properties and corrosion requirements for Upset SCRRequirements Non Sour Service Case1 Sour Service Case 2 Yield Strength0.5% EUL 65000 psi (minimum), 65000 psi (minimum), 80000 psi (maximum)80000 psi (maximum) Ultimate Tensile Strength 0.5% EUL 77000 psi(minimum) 77000 psi (minimum) Yield/Tensile strength ratio 0.89(maximum) 0.89 (maximum) Elongation (% in 2″) 18% (minimum) 18%(minimum) Hardness Vickers (HV10) 269 (maximum) 250 (maximum) Absorbedenergy value for 3 70 minimum Individual, 70 minimum Individual,individual specimens (Joules) 90 minimum Average 90 minimum Average at−30° C. at −40° C. Crack Tip Opening Displacement L- 0.510 minimumIndividual, 0.510 minimum Individual, C direction at −10° C. (mm), 30.635 minimum Average 0.635 minimum Average individual specimens HIC asper NACE TM0284 using CTR 3.0% (maximum) solution “A”. Test period 96hrs. CLR 10.0% (maximum) CSR 1.0% (maximum) FPBT as per ASTM G48, testNo cracks after 720 hrs solution “A” of NACE TM0177. Testing stress 95%of SMYS. Test period 720 hrs.

Table 2 shows a summary of observed microstructures. All microstructuresare homogeneous at midwall, which is the most critical section wheremainly bainite, and a mixture of acicular and non-polygonal ferrite isobserved independent of the section (pipe body, transition or upset).There is a slight presence of martensite close to the OD and IDsections.

TABLE 2 Microstructure of the Upset SCR Seamless Steel Tube Pipe BodyTransition Upset ID Bainite, Tempered Bainite, Tempered MartensiteBainite and presence of Martensite and Acicular and Acicular Ferriteacicular and non-polygonal Ferrite Ferrite MW Bainite and presence ofBainite, Acicular and non- Bainite and Acicular and acicular andnon-polygonal polygonal Ferrite non-polygonal Ferrite Ferrite ODBainite, Tempered Bainite, Tempered Martensite Bainite and presence ofMartensite and Acicular and Acicular Ferrite acicular and non-polygonalFerrite Ferrite

A specific alloy design is developed and heat treatment parameters areset to obtain the desired microstructural characteristics in both pipebody and heavy wall upset sections. The combination of the abovementioned parameters results in excellent mechanical properties whichmeet the strength and corrosion objectives.

The present invention also describes a method for manufacturing aseamless steel tube for steel catenary riser with upset ends having ayield strength at least of 65000 psi both in the pipe body, transitionand the upset-zone comprising the steps of: (a) providing a steel tubecomprising in weight per cent, carbon 0.04-0.10, manganese 0.40-0.70,silicon 0.15-0.35, chromium 0.40-0.70, molybdenum 0.40-0.70, nickel0.10-0.40, nitrogen 0.008 max, aluminum 0.010-0.045, sulfur 0.005 max,phosphorus 0.020 max, titanium 0.003-0.020, niobium 0.020-0.035,vanadium no more than 0.10, copper 0.20 max, Tin 0.020 max, and carbonequivalent 0.43 max and PCM no more than 0.23; (b) quenching andtempering between 630-710° C.

Multiple steps of upsetting and heating cycles in between each upsettingoperation are used to achieve required thickness at the upset ends foreach dimension (35 mm wall thickness and 53 mm wall thickness for thecase 1 and 2 above mentioned) to obtained final as machined upset endsmentioned above.

Weldability and full fatigue tests are performed to a large number ofpipes to establish fatigue performance. These tests are described asfollows:

Welding Program

The properties of the upset pipes subject to different thermal cyclesinduced by welding operations are evaluated initially by welding on a 35mm wall thickness pipe with the chemistry as the upset ends.

The conditions are summarized in Table 3, and have been applied on awelding bevel configuration as recommended in the standard API RP2Z;Preproduction Qualification for Steel Plates for Offshore Structures[1].

This specific welding preparation with one of the bevel at 0° enables toquantify the toughness (impact and CTOD testing) of the HAZ inconditions more severe than with the conventional V or U bevel (thefatigue crack is placed in the prescribed coarse-grain HAZ material forat least 15% of the central two thirds of the specimen thickness).

TABLE 3 API RP2Z welding conditions API RP2Z welding conditions on 35 mmthick pipe X70 Heat Input Preheat Interpass Temp. Interpass Temp.(kJ/mm) Temp. (° C.) (° C.) Fill passes (° C.) Cap passes 0.6 100 100100 0.6 200 200 200 0.8 150 150 150 0.8 200 200 200 0.8 200 200 250 2.0250 250 250 3.0 250 250 250

The weldability test requires characterization of the HAZ for two casessubject to different combinations of heat using an API RP2Z bevel:Preproduction Qualification for Steel Plates for Offshore Structures[8]. All tests passes or exceeds the requirements including hardnessHV10 below 250 for the sour service case (Case 2).

The HAZ characterization has been run on both upset pipes with 28 mm and45 mm thickness at the upset ends, with the welding conditions listed intable 4. The consumables and heat input used are:

-   -   Lincoln STT for the root pass, heat input 0.55-0.75 kJ/mm,    -   P-GMAW for fill and cap passes with heat input 0.6 kJ/mm,    -   SAW for fill and cap passes with heat input equal or greater        than 0.8 kJ/mm.

TABLE 4 API RP2Z welding conditions on both upset pipes X65 API RP2Zwelding conditions on both upset pipes X65 Heat Input Preheat InterpassTemp. Interpass Temp. (kJ/mm) Temp. (° C.) (° C.) Fill passes (° C.) Cappasses 0.8 200 200 250 1.5 250 250 250 3.0 250 250 250

HAZ Characterization: Testing and Results

-   -   Hardness:

On the 35 mm thick pipe, hardness indentations in HAZ are located inlines parallel to the pipe body, at 1.5 mm from inner and outer diameterof pipe and each 4 mm across the thickness.

To meet the requirements of 250 Hv10 maximum in HAZ (from root to cap)for sour service application, the welding conditions are:

-   -   a heat input of minimum 0.65 kJ/mm combined with a preheat        temperature of 200° C. for root pass,    -   a heat input of minimum 0.8 kJ/mm combined with an interpass        temperature of 200° C. for fill passes,    -   a heat input of minimum 0.8 kJ/mm combined with an interpass        temperature of 250° C. for cap passes.

In addition, for the capping, the last bead is not on a side of a bevelbut is deposited within the width of the weld preparation so that eachcap passing at the edges of the bevel gets the benefit of a temperingeffect of the subsequent cap passes. On the upset ends 28 and 45 mmthick, by applying the above mentioned welding conditions, the hardnessin HAZ does not exceed 250 HV10.

The typical macro sections of the API RP2Z welds produced for two heatinputs are shown in FIG. 3. These welds are then tested for hardness andtoughness (Charpy and CTOD) properties.

-   -   Toughness:

Impact testing is run at −40° C., from fusion line+1 mm to fusion line+3mm, on welds run on 35 mm thick pipe and high heat input (2 and 3kJ/mm). Absorbed energy obtained is above 200 J for each specimen.

On the upset ends welds, these very high absorbed energy values in HAZare duplicated, regardless of the wall thickness and the weldingconditions: minimum value achieved 200 J, maximum value achieved 450 J.

In addition, CTOD testing (SENB, Bx2B specimens) in HAZ as per API RP2Zis run at −10° C. With the range of heat input from 0.8 to 1.5 kJ/mm,which is typical of field welding, the achieved CTOD values are from 1.0to 1.5 mm, which shows an excellent ductility of the HAZ.

Development of the Welding Procedure Specification (WPS)

In order to force the fatigue crack to initiate away from the weld areaand so to better quantify the fatigue resistance of the upset design, aspecific welding procedure specification is developed and used for thewelds to be full scale fatigue tested: selection of a welding consumablewith very high toughness, removal of weld root and reinforcement of cap.

Full scale testing shows excellent fatigue behavior of upset girthjoints. In both cases, the data correspond to failure, or run-outs, wellbeyond the target mean curve for the sets of tests, demonstrating thatboth geometries of girth welded upset qualify as top class (B1 inDNV-RP-C203) component for fatigue resistance. Mean curve results can beseen in FIGS. 8 and 13 for case 1 and case 2 respectively.

EXAMPLES

Heavy Wall Upset seamless steel tubes with the following characteristicsare used:

Case 1: 273.1 mm OD by 22.0 mm WT Pipe body, 28 mm WT as machined UpsetEnds and 35 mm as upset ends, steel grade X65 for non-sour serviceapplication (10.75″×0.866″)

Case 2: 273.1 mm OD by 31.8 mm WT Pipe body, 45 mm WT as machined UpsetEnds and 53 mm as upset ends, steel grade X65 for sour serviceapplication (10.75″×1.250″).

Case (1) Upset SCR TDZ 10.75″ OD×0.866″ WT X65 Non Sour Service

FIGS. 4( a) and (b) and 5 show the Yield Strength (YS), Ultimate TensileStrength (UTS) and the YS/UTS ratio evaluated at room temperature forquenched and tempered material. Longitudinal and transverse roundspecimens taken from sections representing the Upset End, SlopeTransition and Pipe Body are tested in two quadrants, 0° and 180°. Allspecimens are standard round except by those from the pipe body in thetransverse direction which are sub-size round. FIGS. 4( a) and (b) showall the YS and UTS values obtained from the tensile test in thelongitudinal and transverse directions, respectively.

FIGS. 4( a) and (b) show that all Yield Strength values obtained areabove 65,000 psi minimum and do not exceed the 80,000 psi maximum. Allthe Ultimate Tensile strength values obtained are above 77,000 psiminimum established.

FIG. 5 shows that, for the YS/UTS ratio, all the values are below 0.89which is established as maximum YS/UTS specification. The values ofYS/UTS ratio are shown in FIG. 5 for both the longitudinal andtransverse directions.

Hardness Test

For the material in the “as quenched and tempered condition”, HardnessVickers HV10 (3 readings per row) are measured on the OD, MW and IDsections in 4 quadrants taken from the Upset End, Slope Transition andPipe Body regions. The hardness readings are taken at 1.5 mm from outerdiameter (OD) and inner diameter (ID). As quenched and tempered materialHV10 test results are shown in FIG. 6.

Even as the material from case 1 is not initially considered for sourservice application, as shown in FIG. 6, all hardness readings are below250HV10 (22 HRc) complying with NACE requirement for material to be usedin sour environments.

Toughness Test

The fracture mechanics characteristic is evaluated using the TransverseCharpy V Notch Impact Test. The test temperature is −30° C. Sets ofthree full size specimens (10×10 mm) are taken from upset end, slopetransition and pipe body regions in two quadrants, 0° and 180°, for eachsample of quenched and tempered material.

FIG. 7 shows that all the individual values of Absorbed Energy are above70 Joules which is established as minimum target and 90 Joules asminimum average of 3 specimens. The transition temperature obtained inthe transverse direction using Charpy V-notch 10×10 specimens inmaterial representing pipe body and upset end are below −60° C. as isshown in Tables 5 (a) (b).

TABLE 5 Transition Temperature Curve. (a) Pipe body and (b) Upset EndTransverse Charpy V-Notch test results (Joules), 10 × 10 mm specimenAverage Average Test Absorbed Shear Temperature % % % Energy Area SamplePipe/End Location ° C. 1 Sh. A 2 Sh. A 3 Sh. A (J) (%) (a) 64283 Pipe 10Pipe −30 439 100 419 100 443 100 434 100 North body −40 440 100 408 100415 100 421 100 End −50 435 100 355 100 437 100 409 100 −60 357 100 451100 280 100 363 100 (b) 64286 Pipe 15 Upset −30 425 100 428 100 431 100428 100 South End −40 388 100 374 100 435 100 399 100 End −50 424 100422 100 435 100 427 100 −60 344 100 384 100 394 100 374 100

CTOD results representing the pipe body and upset end, as own in Table6, show exceptional results above 0.6 mm at −30° C.

TABLE 6 CTOD Results Representing (a) Pipe Body (b) Upset End TestAverage Minimum Temperature Delta (mm) CTOD Delta CTOD Delta Sample PipeEnd ° C. 1 2 3 Value (mm) Value (mm) PIPE BODY - CTOD TEST RESULTSLONGITUDINAL ORIENTATION RECTANGULAR BX2B SPECIMEN (a) 64283 10 North−10 1.54 1.51 1.49 1.51 1.49 64286 915 South −30 1.49 1.52 1.39 1.471.39 Minimum Specification 0.635 0.510 UPSET END - CTOD TESTS RESULTSLONGITUDINAL ORIENTATION COMPACT BX2B SPECIMEN (b) 64283 10 North −101.13 1.11 1.10 1.11 1.10 64286 15 South −30 1.15 1.11 1.13 1.13 1.11Minimum Specification 0.635 0.510

Microstructural Analysis

Samples from as-quenched and as-quenched and tempered material areprepared for microstructural analysis. The transverse face to therolling axis is metallographically prepared by sanding down to 600 sandpaper and polished to a mirror-like appearance with diamond paste andetched with Nital at 2% to carry out microstructural observations byoptical microscope.

Microstructures are observed on OD, MW and ID sections of pipe body,slope transition and upset end regions. Two quadrants, 0° and 180°,photomicrographs at 500× representing the microstructure from OD, MW andID are obtained.

In this case, the observed microstructure in the pipe body afterquenching consists of a mixture of predominantly bainite and acicularferrite through the wall thickness and a slight presence of martensiteclose to the outer and inner surface. Similarly, bainite and acicularferrite and some regions of non-polygonal ferrite are observed throughthe wall thickness at the upset section.

The prior austenitic grain size (PAGS) are measured using image analysison as-quenched material etched with saturated aqueous picric acid onsamples from the pipe body and the upset end at 0° and 180° Quadrants,resulting in an average size of 9/10 ASTM.

The microstructure after the tempering treatment consists ofpredominantly bainite and acicular ferrite are observed through the wallthickness on material representing pipe body, slope transition and upsetend.

Fatigue Test Results

The fatigue test results are shown in FIG. 8. The test results show veryhigh fatigue performance at upset ends, transition and pipe body.

Case (2) Upset SCR TDZ 10¾″ OD×1.250″ WT X65 Sour Service

For case (2), in addition to all the destructive testing includingtensile, hardness toughness performed in case (1), the Sour ServiceHydrogen Induced Cracking Test as per NACE TM0284 and Sulphide StressCracking by using the Four Point Bend Test is performed. FIG. 9 showsthe tensile results where it can be seen that Yield Strength valuesobtained are above 65,000 psi and do not exceed the maximum value of80,000 psi. All the Ultimate tensile strength values obtained are above77,000 psi which is established as the minimum specified.

All the YS/UTS ratio values are below 0.89 as shown in FIG. 10 for bothlongitudinal and transverse direction.

As shown in FIG. 11, all the hardness readings are below 250 HV10 (22HRc) complying with NACE MR0175 requirements for materials to be used insour environments.

For this case (2), the Charpy test temperature is −40° C. Sets of threefull size specimens (10×10 mm) are taken from midwall of upset end,slope transition and pipe body regions in two quadrants 0° y 180°, fromquenched and tempered material. As shown in FIG. 12, all results areabove the expected minimum absorbed energy values of 70 Joules minimumindividual and 90 Joules as minimum average of 3 specimens.

Transverse Charpy V-Notch impact transition curves are obtained from 2samples, one representing upset end and another one representing pipebody from quenched and tempered material for each case.

The transition temperature obtained in the transverse direction usingCharpy V-notch 10×10 specimens is between −50° C. and −60° C. for thematerial representing the upset end and below −70° C. for materialrepresenting pipe body as shown in Table 9.

As shown in FIG. 12, all results were above the expected minimum valuesof 70 Joules minimum individual and 90 Joules as minimum average of 3specimens.

Transverse Charpy V-Notch impact transition curves are obtained from 2samples representing upset end and another representing pipe body ofquenched and tempered material for each case.

The transition temperature obtained in the transverse direction usingCharpy V-notch 10×10 specimens is between −50° C. and −60° C. for thematerial representing the upset end and below −70° C. for materialrepresenting pipe body as shown in Table 7.

TABLE 7 Transition Temperature Curve. (a) Pipe body and (b) Upset EndTransverse Charpy V-Notch test results (Joules), 10 × 10 mm specimenAverage Average Test Absorbed Shear Temperature % % % Energy Area SamplePipe/End Location ° C. 1 Sh. A 2 Sh. A 3 Sh. A (J) (%) (a) 64521 Pipe 10Pipe −20 436 100 450 100 439 100 442 100 North body −30 432 100 440 100422 100 431 100 End −40 434 100 442 100 446 100 441 100 −50 446 100 436100 449 100 444 100 −60 384 100 440 100 439 100 421 100 −70 398 100 424100 435 100 419 100 (b) 64518 Pipe 6 Upset −20 422 100 447 100 409 100426 100 North End −30 430 100 430 100 452 100 437 100 End −40 429 100428 100 424 100 427 100 −50 443 100 449 100 405 100 432 100 −60 9 0 439100 432 100 294 67 −70 6 0 415 100 444 100 288 67

CTOD results from material representing pipe body and upset end areabove 0.6 mm at −10° C. as shown in Table 8.

TABLE 8 CTOD Results Representing (a) Pipe Body (b) Upset End TestAverage Minimum Temperature Delta (mm) CTOD Delta CTOD Delta Sample PipeEnd ° C. 1 2 3 Value (mm) Value (mm) PIPE BODY - CTOD TEST RESULTSLONGITUDINAL ORIENTATION BX2B SPECIMEN (a) 64577 9 North −10 1.50 1.551.54 1.53 1.50 64578 9 South −10 1.61 1.56 1.57 1.58 1.56 MinimumSpecification 0.635 0.510 UPSET END - CTOD TESTS RESULTS LONGITUDINALORIENTATION COMPACT SPECIMEN (b) 64577 9 North −10 1.13 1.14 1.07 1.111.07 64578 9 South −10 1.14 1.15 1.17 1.15 1.14 Minimum Specification0.635 0.510

Corrosion Test Only Case (2)

Hydrogen Induced Cracking

HIC test is performed on 1 sample representing upset end and anotherrepresenting pipe body for Case 2. Each set of 3 specimens (3 quadrants,0°, 120° and 240°) representing pipe body and another set representingupset end is tested as per NACE TM0284 using Solution “A”, test periodwas 96 hours. The results are shown in Tables 9 and 10.

TABLE 9 Hydrogen Induce Cracking Test Results - Pipe Body HYDROGENINDUCE CRACKING TEST RESULTS - PIPE BODY SOLUTION A: NACE TM 0284 CrackSection Average Length Width Cracking Coupon Average Cracking SpecimenSection (mm) (mm) Blisters % CSR % CLR % CTR % CSR % CLR % CTR 1 A1 0.000.00 None 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 A2 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 A3 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 2 B1 0.00 0.00 None 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 B2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 B3 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 3 C1 0.00 0.00 None 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 C2 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 C3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

TABLE 10 Hydrogen Induce Cracking Test Results - Upset End HYDROGENINDUCE CRACKING TEST RESULTS - UPSET END SOLUTION A: NACE TM 0284 CrackSection Average Length Width Cracking Coupon Average Cracking SpecimenSection (mm) (mm) Blisters % CSR % CLR % CTR % CSR % CLR % CTR 1 A1 0.000.00 None 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 A2 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 A3 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 2 B1 0.00 0.00 None 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 B2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 B3 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 3 C1 0.00 0.00 None 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 C2 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 C3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

These tables show that neither cracks nor blisters are found after testperiod showing that all the requirements for the Hydrogen InducedCracking test are met.

Sulfide Stress Cracking

SSC Four Point Bend Test is performed on 1 sample representing upset endand another representing pipe body. Each set of 3 specimens (3quadrants, 0°, 120° and 240°) representing pipe body and another setrepresenting upset end is tested as per ASTM G48. Test solution “A” ofNACE TM0177 is considered. Testing stress is 95% of Specified MinimumYield Strength (SMYS) and two test periods of 96 hours and 720 hours.The results are shown in Tables 11 and 12.

TABLE 11 SSC Four Point Bend Test Results Representing Material FromPipe Body (a) After 96 Hrs. Exposure (b) After 720 Hrs. Exposure StressSpecimen Initial Values Final Values Applied No. SATi PHi SATf pHf %SMYS Result SULFIDE STRESS CRACKING - FOUR POINT BEND TEST SOLUTION “A”NACE 0177-96 - TEST DURATION: 96 HRS. PIPE BODY (a) 1 2418.32 2.722503.61 3.57 95 Not failed 2 2418.32 2.72 2503.61 3.57 95 Not failed 32418.32 2.72 2503.61 3.57 95 Not failed SULFIDE STRESS CRACKING - FOURPOINT BEND TEST SOLUTION “A” NACE 0177-96 - TEST DURATION: 720 HRS. PIPEBODY (b) 1 2809.95 2.70 2980.25 3.62 95 Not failed 2 2809.95 2.702980.25 3.62 95 Not failed 3 2809.95 2.70 2980.25 3.62 95 Not failed

TABLE 12 SSC Four Point Bend Test Results From Upset End (a) After 96Hrs. Exposure (b) After 720 Hrs Exposure Stress Specimen Initial ValuesFinal Values Applied No. SATi PHi SATf pHf % SMYS Result SULFIDE STRESSCRACKING - FOUR POINT BEND TEST SOLUTION “A” NACE 0177-96 - TESTDURATION: 96 HRS. UPSET END (a) 1 2418.32 2.72 2503.61 3.57 95 Notfailed 2 2418.32 2.72 2503.61 3.57 95 Not failed 3 2418.32 2.72 2503.613.57 95 Not failed SULFIDE STRESS CRACKING - FOUR POINT BEND TESTSOLUTION “A” NACE 0177-96 - TEST DURATION: 720 HRS. UPSET END (b) 12809.95 2.70 2980.25 3.62 95 Not failed 2 2809.95 2.70 2980.25 3.62 95Not failed 3 2809.95 2.70 2980.25 3.62 95 Not failedTables 11 and 12 show that all Four Point Bend specimens passedsuccessfully the SSC test after the test period, stressed at 95% SMYS,no cracks are observed after 96 hours and even after 720 hours.

Microstructural Characterization

Optical Microscopy and Scanning Electron Microscopy is used for materialcharacterization. Microstructural analysis is performed on OD, MW and IDsections of pipe body, slope transition and upset end regions in twoquadrants 0° and 180° for samples in the as-quenched condition andquench and tempered condition.

The pipe body as-quenched microstructure consists of predominantlybainite and acicular ferrite at midwall and, close to the outer andinner surface, a slight presence of martensite is observed.

The upset as-quenched microstructure consists of predominantly bainiteand acicular ferrite through the wall thickness.

The PAGS is measured using image analysis on as-quenched material etchedwith saturated aqueous picric acid on samples from pipe body and upsetend. An average PAGS size of ⅞ ASTM is obtained for both pipe body andupset end respectively.

The microstructure at mid-wall after tempering consists of predominantlybainite and acicular ferrite at the pipe body and slope transition; andbainite, acicular ferrite, and non-polygonal ferrite at the upset ends.

Fatigue Results

The fatigue test results are plotted in FIG. 13. The test results showvery high fatigue performance at upset ends, transition and pipe body.

The invention has been fully described and experimental fatigue resultsobtained shows that the fatigue performance for these two UpsetSolutions described above in case (1 and 2) has been increased with afactor ranged between 3 and 15.

1. A seamless steel pipe for a steel catenary riser with upset ends comprising in weight per cent, carbon 0.04-0.10, manganese 0.40-0.70, silicon 0.15-0.35, chromium 0.40-0.70, molybdenum 0.40-0.70, nickel 0.10-0.40, nitrogen 0.008 max, aluminum 0.010-0.045, sulfur 0.005 max, phosphorus 0.020 max, titanium 0.003-0.020, niobium 0.020-0.035, vanadium no more than 0.10, copper 0.20 max, tin 0.020 max, and carbon equivalent 0.43 max and PCM no more than 0.23 and having a yield strength of at least of 65000 psi, the ultimate tensile strength of at least 77000 psi and YS/UTS ratio below 0.89 in material representing the pipe body, the transition zone and the upset end.
 2. The seamless steel pipe according to claim 1, in which the microstructure of as-quench and temper material is homogeneous at midwall which is the most critical section and it is mainly bainite and a mixture of acicular and non polygonal ferrite independently of the section: pipe body, transition or upset zones.
 3. The seamless steel pipe according to claim 1, in which the prior austenitic grain size has an average size is at least 7 ASTM both in pipe body and in the upset pipe ends.
 4. The seamless steel pipe according to claim 1, which the material in the as quenched and tempered condition has a Hardness Vickers HV10 value below 250 both in the pipe body and the upset ends.
 5. The seamless steel pipe according to claim 1, in which the materials from pipe body and upset ends have an individual value of absorbed energy above 70 Joules, and 90 Joules average of three specimens and the transition temperature in the transverse direction was below −50° C.
 6. The seamless steel pipe according to claim 1, in which the materials from pipe body and upset ends exceed in at least 2 times the minimum individual value of 0.51 mm required of crack tip opening displacement test (CTOD test).
 7. The seamless steel pipe according to claim 1, which is weldable at the upset ends in a heat input range between 0.8 KJ/mm and 1.5 KJ/mm, where CTOD testing using SENB, Bx2B specimens undertaken from the heat affected zone run at −10° C. as per API RP2Z, gave CTOD values above 0.6 mm.
 8. The seamless steel pipe according to claim 7, which is weldable at the upset ends in a heat input between 0.8 KJ/mm and 3.0 KJ/mm and the hardness in the heat affected zone is less than 250 HV10.
 9. The seamless steel pipe according to claim 7, which is weldable at the upset ends in a heat input between 0.8 KJ/mm and 3.0 KJ/mm and the absorbed energy values evaluated at fusion line +1 mm in the heat affected zone are above 100 Joules.
 10. The seamless steel pipe according to claim 1, which is weldable at the upset ends in a heat input range between 0.8 KJ/mm and 1.5 KJ/mm, where CTOD testing using SENB, Bx2B specimens undertaken from the axis of the weld metal run at −10° C. gave CTOD values above 0.6 mm
 11. A method for manufacturing a seamless steel tube for steel catenary riser with upset ends having a yield strength at least of 65000 psi both in the pipe body, transition and the upset-zone comprising the steps of: (a) providing a steel tube comprising in weight per cent, carbon 0.04-0.10, manganese 0.40-0.70, silicon 0.15-0.35, chromium 0.40-0.70, molybdenum 0.40-0.70, nickel 0.10-0.40, nitrogen 0.008 max, aluminum 0.010-0.045, sulfur 0.005 max, phosphorus 0.020 max, titanium 0.003-0.020, niobium 0.020-0.035, vanadium no more than 0.10, copper 0.20 max, tin 0.020 max, and carbon equivalent 0.43 max and PCM no more than 0.23; (b) upsetting the tube ends in multiple steps with intermediate heating cycles in between to achieve the required thickness (c) quenching and tempering between 630-710° C.; (d) machining the upset ends.
 12. The method for manufacturing a seamless steel pipe according to claim 11 wherein pipes were hot rolled using a recrystallization controlled rolling scheme, manufactured from round billets obtained by continuously cast (CC) process.
 13. The method for manufacturing a seamless steel pipe according to claim 11 wherein the pipes were upsetted by reheating the pipe ends above the dissolution temperature of Nb (C, N) to provide adequate plastic flow during each upset operation whilst controlling austenite grain size by precipitation of fine TiN particles.
 14. A pipe string for use as steel catenary riser for non-sour service environment using the pipes according to claim 1, wherein pipes are welded on the upset ends.
 15. A pipe string for use as steel catenary riser for sour service environment using pipes according to claim 1, wherein pipes are welded on the upset ends. 